Roughly 30 years ago, Bomag pioneered some of the first cold
in-place asphalt recyclers in the roadbuilding industry. This was not only a
new piece of equipment, it was the introduction of a new concept. The idea was
intriguing enough. Instead of investing the time, effort and materials in
totally reconstructing an asphalt road from the ground up, cold in-place
asphalt recycling would permit the reuse of the previous road surface and
sub-base to create a strong foundation for a new roadway. Though cold in-place
recycling offered a revolutionary alternative that promised a tremendous cost
savings, contractors, county engineers and state officials alike were
skeptical. How could something that costs less be just as effective as the
established reconstruction method? The task of selling the concept proved to be
an uphill battle.

Today--30 years later--several manufacturers have introduced
cold in-place asphalt recycling equipment. Though credibility has been
established through education and successful applications in many states and
counties, understanding the process and its benefits remains the key challenge
to its acceptance as a viable road reconstruction option.

Before you begin . . .

It's difficult to describe a "typical" cold
in-place recycling application since there are so many variables to consider.
What materials currently make up the structure of the road? Is the current
subgrade satisfactory, or will the road require full-depth reclamation? Does
aggregate or asphalt need to be added to the mix? What about additives? All of
these questions, and several others, need to be answered before recycling can
take place. But there are basic steps that should be conducted with each
application.

Core samples

In order to improve a road through cold in-place recycling,
one first needs to know what materials make up the current road surface and
base. This is why core sampling is the single most important step in the
recycling process.

Core sampling takes place before any equipment is on site.
It determines the composition of the asphalt in the road, the percentage of
asphaltic concrete in the mix and the amount and type of aggregate present, as
well as pinpointing any potential problems that can't be seen on the surface.
Though many will claim to know enough about road construction or a particular
road surface to forgo taking core samples, sampling is truly the only way to
successfully determine the recycling needs. Since the success or failure of a
road relies on the makeup of the base materials, such information should not be
left to speculation.

When taking core samples, it's best to take samples from
both the best and worst portions of the road. As those in the road construction
industry know, a core sample taken from one area and another taken a mile down
the road can yield two completely different results. One must analyze all the
core samples and recycle based on an average of the measurements.

Grading

Once the core samples have been taken and the asphalt and
base makeup is determined, it's time to put the recycling plan into action. But
before beginning to grind the road, the contractor should take time to evaluate
the current road width. Many of today's state highways are simply not built to
the exact specifications. When embarking on a recycling project, it may make
sense to grind a bit wider than the existing road to effectively stabilize the
base under what will be the new road. In exercising this option, first utilize
a motor grader to scrape the shoulders free of any organic material. This step
should not be taken lightly. Any organic material that gets into the mix will
eventually die. Following decay of the material, a void is left in the road's
substructure resulting in a possible collapse and destruction of the surface.
By simply making a few passes with a motor grader to skive off any organic
material, much of this concern can be eliminated.

Recycling

Once the road's needs have been determined and all organic
material has been removed, it's time to grind, or recycle, the road. This
process involves grinding both the asphalt and aggregate base together. Again,
much of the specifics of this process depend on the core samples. If it is
found that the established base is in good shape, one may decide that grinding
only a minimal amount of the base material is necessary. On the other hand, if
the subgrade is weak, aggregate can be added prior to recycling to establish a
more effective material gradation. Since the most common cause of road failure
is a faulty base, full-depth reclamation may be a consideration.

Basically, the cold in-place asphalt recycler uses a
spinning rotor with cutting teeth to pulverize the road and base material,
leaving the ground-up material in its place. The time it will take for this
initial cut will depend on the makeup and hardness of the material, but the
core samples should provide a clue. If core samples show that the base is made
up of fine materials, aggregate should be added to the surface of the road
prior to grinding. When recycling, material should be pulverized until the
proper grade is established. Sometimes this is achieved with the first pass;
other times multiple passes may be necessary.

With basic mechanical stabilization, a water truck is
utilized following the recycler to maintain moisture in the ground-up material,
acting as a lubricant to facilitate the best compaction. In other cases, the
use of an additive may be necessary to help stabilize the material.

Additives

Additives serve many purposes. As with water, some additives
act as a lubricant to facilitate compaction. In other cases, they may act as a
binder for the material, helping to make the road impermeable to water. Since
water is the natural enemy to the structural integrity of any road,
impermeability is a key objective.

Compaction

Following the establishment of a proper grade through
grinding and the addition of water or additives, the material is ready for
compaction. The best tool to use for this step is a padfoot roller.

Padfoot rollers are ideally suited for the compaction of
cohesive soils. Because of the basic properties of asphalt and its presence
within the mix, the material tends to have the properties of a cohesive soil.
Padfoot rollers work well because they compact unevenly, preventing material
layering and establishing a good mix of material.

During the first pass, the pads of the roller typically sink
all the way down to the drum. But as compaction increases, the pads begin to
"walk out" or rise up out of the mix as the material achieves
density. Eventually, the roller will be standing near the tips of the pads,
indicating that proper compaction has been established. It is important to
remember that if the material becomes mis-shapen during the compaction process,
the use of a motor grader is advised for any required reshaping. Additionally,
when compaction is established, the grader should be used to remove any of the
"uncompactable" materials, or ravel, from the surface. Finally, a
rubber-tired or steel-drum roller is used to smooth and finish the recycled
base.

Sealing

After the road has been mixed, shaped and compacted, it
should then be protected by some form of temporary seal. A fog seal is an
asphalt emulsion that is applied to the top of the base and allows water to
shed off the top instead of soaking in and compromising the integrity of the
compacted base.

After the sealant has been applied, the newly established
base is ready for an overlay, chipseal or whatever is specified for the job. If
an additive was not used during recycling, the finishing treatment can be
applied shortly after completion. If an additive was used, time must typically
be allowed for the substance to cure. This extra time needed between recycling
and the finishing treatment makes the sealing step even more important to
maintain the integrity of the compacted material.

Repeat savings

So if one is already familiar with conventional
reconstruction, why even consider a "new" process, such as cold
in-place recycling? The answer is simple--money.

Cold in-place recycling, which is essentially total
reconstruction of a road, encounters few cost-prohibitive problems. Material
costs are less because the existing material is recycled and reused. Though
many times additional aggregate or asphalt must be added to create a proper
base, this is much more cost-effective than removing the road bed.
Additionally, the material is recycled "in-place," meaning there is
little need for excavation or hauling.

In the end, cold in-place recycling costs anywhere from
one-third to one-half of the total cost incurred for conventional
reconstruction. And when done properly, cold in-place recycling has offered
better results than conventional reconstruction.

Don't miss the concept

So if it yields equivalent or better construction quality,
yet costs substantially less, one would think that cold in-place recycling
would be mainstream. The fact is that many still do not even consider the
process.

One obstacle is the perception that cold in-place recycling
only involves the road's top layer of asphalt and, thus, cannot be directly
compared to conventional construction methods. Though this misconception has
been mostly overcome in the last 30 years, it should be reiterated that cold
in-place recycling is total reconstruction of a road. Because the process goes
down to the subgrade, the two methods are quite comparable.

Other misconceptions stem from the lower cost of recycling.
Many contractors and government agencies don't believe that something that
costs less can actually do the same or better job than conventional methods.
The belief that spending more money results in a better quality product is
still quite prevalent.

Additionally, the fact that recycling is still perceived as
a new process is another problem. There is nothing more difficult than trying to
convince someone that something new could be better when an established method
is the norm.

Additives also have been a point of contention in the
acceptance of recycling. There are a large number of additives on the
market--some good, some bad--and many times they are sold as being able to do
something they cannot. When the wrong additive or incorrect dilution is used,
the entire recycling process is jeopardized. If this occurs, it's often the
process that receives the blame, not the additive.

The only true limitations to recycling are concrete and
brick. In some cases, asphalt roadways have bases constructed of concrete or
are overlays of previous brick streets. A cold in-place recycler cannot handle
such materials. Again, a core sample would indicate the presence of such
materials, but this would be the one situation where excavation and
conventional reconstruction would be the only option.

Accepted everywhere?

Though the cold in-place recycling process may appear too
good to be true--offering the benefits of conventional reconstruction at a
fraction of the cost--many counties and states have experienced tremendous
success with the recycling option. Acceptance is still a work in progress, but
when twice as many roads are being rebuilt for the same taxpayer's dime, the
results are becoming harder to ignore. As more and more decision makers learn
about and gain confidence in the process, cold in-place recycling will steadily
win acceptance.